1. Field of the Invention
The present invention relates to an ophthalmologic apparatus which obtains unique information on a subject's eye (e.g., eye refractive power, a cornea shape, and a fundus image) or alignment information.
2. Description of the Related Art
An eye refractive power measuring apparatus is conventionally known as an ophthalmologic apparatus, which projects an index on a fundus of a subject's eye and then measures spherical refractive power, astigmatic refractive power, and an astigmatic axial angle of the subject's eye based on the reflected fundus image. In general, the eye refractive power measuring apparatus projects an index on a fundus of a subject's eye, forms and detects reflected light from the fundus as a ring image on a two-dimensional sensor therein, and then calculates and outputs a radius of the ring image as the spherical refractive power, and distortion of the ring image as the astigmatic refractive power.
In recent years, optical systems in the eye refractive power measuring apparatus have become complex along with downsizing of the apparatus. It has become common that a plurality of optical systems is combined or divided and the optical system on the exit side of an objective lens on the subject side is shared in the apparatus.
Examples of a plurality of optical systems in the eye refractive power measuring apparatus, other than an eye refractive power measuring optical system are as follows. An anterior eye observation optical system is used for selecting and confirming a measurement range. An alignment optical system is used for aligning the apparatus and a subject's eye. A fixation target optical system guides fixation and focusing state of a subject's eye by using an index in the apparatus. Further, there is a cornea shape measuring optical system for measuring a cornea shape, depending on the product configuration.
The ophthalmologic apparatus discussed in Japanese Patent No. 3927873 includes a dichroic mirror which is arranged near the exit side of an objective lens and used as a beam splitting member in a light receiving system and as a beam combining member in a light projecting system. The dichroic mirror transmits a wavelength used for a transmission optical system and reflects a wavelength used for a reflection optical system, and the optical system on the exit side of an objective lens is shared in the apparatus. An eye refractive power measuring optical system is used as the transmission optical system. An observation optical system, a fixation target optical system, and a cornea shape measuring optical system are used as the reflection optical system.
The dichroic mirror which is employed for transmitting or reflecting a wavelength is generally a parallel plate glass having a thickness of several millimeters. The dichroic mirror is arranged to be tilted by approximately 45 degrees with respect to an optical axis in a measurement optical path (i.e., an optical path of a light receiving system which receives reflected light from a fundus).